You probably need to create some java utility class which could get sitename iterate though it's all documents of document library and calculate total size occupied.This is really a very useful usecase but I am not able to recollect any out of box feature though which this can be achieved.Can anybody else share their thoughts on that?

Apparent variable 'size' was found in a static scope but doesn't refer to a local variable, static field or class. Possible causes:You attempted to reference a variable in the binding or an instance variable from a static context.You misspelled a classname or statically imported field. Please check the spelling.You attempted to use a method 'size' but left out brackets in a place not allowed by the grammar. at line: 77, column: 21

Apparent variable 'size' was found in a static scope but doesn't refer to a local variable, static field or class. Possible causes:You attempted to reference a variable in the binding or an instance variable from a static context.You misspelled a classname or statically imported field. Please check the spelling.You attempted to use a method 'size' but left out brackets in a place not allowed by the grammar. at line: 77, column: 28